1. Field of the Invention
This invention relates to a method for providing an improved sequence of pulse control for Nuclear Magnetic Resonance imaging. More particularly, the invention concerns an improved method of obtaining a multi-slice acquisition in which images from several slices are simultaneously acquired at preselected repetition times.
2. Discussion of Background
Nuclear magnetic resonance (NMR) has recently become extremely popular for the reproduction of images of the interior portions of the human body for diagnostic purposes. Although the examination of the interior body or a body portion having a variety of relatively soft tissues is the primary use for NMR techniques, they can also be utilized for circumstances whereby characteristics of tissues under inspection can be portrayed which are ordinarly not readily obtained using a radiography or other imaging techniques.
The fundamental description of basic NMR physics and imaging techniques is found in Kaufmann et al., Nuclear Magnetic Resonance Imaging in Medicine, IGAKUSHOIN Medical Publishers, Inc., New York and Tokyo (1981). The important features of the NMR technique which are commonly used in previous methods will now be elaborated upon.
The body under examination is subjected to one magnetic field which is usually constant in magnitude and another field which lies along at least one different vector from the first magnetic field. The second magnetic field is usually time-varying with the exact characteristic of each of these fields being a function of choice between any one of several available imaging techniques. During NMR examinations, the characteristics of the field energy must be pre-selected. As an example, one of the techniques called a spin-echo imaging, utilizes a pre-selected repetition time T.sub.R, which is defined as the time between successive application of the same pulse sequence. Also pre-selected is the sampling delay time .tau. (TAU) which is often set equal to TE/2 where TE is the time at which a spin-echo is measured. The first and second fields are applied to the tissue being examined in accordance with selected times and the results are detected and stored with the levels of the storage signals being correlated with their physical positions and with these levels being represented by a matrix of numbers. The numbers of the matrix are then displayed as a matrix of points of pixels which have different light or dark levels and the composite of these pixels forms an image having various contrasting areas.
A physician utilizes the results of these contrasting areas within the image to observe and analyze a "slice" of the body of which the image was made from and, in a medical context, to thereby form a diagnosis. The degree of contrast between the various areas is a function of T.sub.R and .tau. values which are selected before the measurement is made. The degree of contrast is also a function of the intrinsic properties of the materials including the net magnetization M.sub.O (which is proportional to Proton density), and the relaxation times T1 and T2. While one particular set of values for T.sub.R and .tau. will produce an image with excellent contrast between certain sets of materials, it must be noted that this contrast will probably be insufficient between other types of materials being used. As a result of this varying contrast which depends upon the materials being used, it is necessary to make numerous sets of measurements with various values for T.sub.R and .tau.. The images which result from these measurements use these different values of T.sub.R and .tau. in order to adequately examine various tissues involved.
This time consuming process in which the patient is often subjected to discomfort and sometimes repeatedly subjected to the effects of strong directional magnetic fields has been alleviated in accordance with the method and apparatus disclosed in copending application Ser. No. 727,674 filed Apr. 26, 1985, of which the present inventor is a co-inventor. Utilizing the method and the apparatus of the copending application an apparatus is disclosed which is capable of obtaining intrinsic parameters of the body materials under examination and forming synthetic images based on those intrinsic parameters in order to permit the synthesis of images formed on the basis of other selectable parameters. The technique of the copending application provides a method in which a minimal number of measurements can be made and in which the data derived from these measurements is used to form synthetic images which include tissue contrast of a type which have been produced by measurements made with selected parameters such as T.sub.R and .tau. value other than those which are actually used for the measurements.
The intrinsic parameters are generally found by acquiring NMR signals for several different repetition times TR and applying them as a function of time. The value of the spin lattice relaxation time T1 is a measure of the curvature of a smooth curve which would most closely match the plotted values of measured signals. Because of the statistical uncertainty in the measured signals, there is uncertainty in the computed value of T1 which can be compensated for by taking measurements several times at repetition times initially used or by taking measurements at additional repetition times.
Applicants' co-pending application Ser. No. 824,784 filed Jan. 31, 1986 provides a method and an apparatus for solving algorithms for the spin-relaxation time T1 and the spin-spin relaxation time T2 which are implemented as a sequence of passes through a digital video processor (DVP) which is reconfigured, between each pass, to perform the next desired mathematical step of the algorithm.
Although several acquisition techniques for clinical NMR images are utilized including schemes which acquire images for several echo times TE for several slices there is a problem with respect to patient motion which can occur between the time of scanning for a first repetition time TR1 and a second repetition time TR2 whereby images could be caused to no longer register. Additionally if a short time is used for TR1, the number of slices scanned is potentially limited.
Additional background information and disclosure of devices and techniques in the field to which this invention relates can be found in the following articles and U.S. patents.
F. W. Wehrli, J. R. McFall, and G. H. Glover. The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and operator-selectable parameters presented at the meeting of the SPIE, Medicine XII, volume 419, April 1983.
I. R. Young, et al. Contrast in NMR Imaging. Presented at the Society of Magnetic Resonance in Medicine, August 1983.
D. Ortendahl, et al. Calculated NMR images. Presented at the Society of Magnetic Resonance in Medicine, August 1983.
P. L. Davis, et al. Optimal spin-echo images for liver lesions by retrospective calculations. Presented at the Society of Magnetic Residence in Medicine, August 1983.
T. C. Farrar, et al. Pulse and Fourier Transform NMR. Academic Press, New York, 1971, pp. 22-29 (Multi-echo pulse sequence).
L. E. Crooks, et al. Clinical efficiency of nuclear magnetic resonance imaging. Radiology 146: 123-128 (1983). (Multi-planar multi-echo imaging.)
F. W. Wehril. Magnetic Resonance Tomorrow--Issues and Challenges. General Electric Company Medical Systems, Booklet No. 5917, 1984, p. 10 (Multi-TR multi-echo).
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,789,832 Damadian 4,045,723 Ernst 4,284,948 Young 4,292,977 Krause et al 4,297,637 Crookes et al 4,307,343 Likes 4,318,043 Crookes et al 4,354,499 Damadian 4,355,282 Young et al 4,390,840 Ganssen et al ______________________________________